Carrier reduced modulator



Aug. 15, 1939. G. 1.. USSELMAN CARRIER REDUCED MODULATOR Filed June 26,1937 ELECTROSTATIC sol/R05 or news ENERGY INVENTOR 6.1. USSELMAN BYATTORNEY SIGNAL /4./-T SOURCE HF l SOURCE OF WAVE ENERG Y Patented Aug.15, 1939 UNITED STATES PATENT OFFICE CARRIER REDUCED MODULATORApplication June 26, 1937, Serial No. 150,499

3 Claims.

This application includes a new and improved means for producingsideband modulation with the carrier reduced or suppressed, themodulation being amplitude or phase and amplitude.

In modulators of the carrier reduced or suppression type knownheretofore such as shown in Carson Patents a l-"1,343,306 and#1,343,307, two balanced tubes have been used in the modulator. In thepresent invention by the use of a novel circuit arrangement I accomplishsideband modulation by the use of a single tube in which modulation thecarrier is reduced or entirely suppressed. In a modification, Iaccomplish sideband modulation accompanied by phase modulation withcarrier reduction or suppression.

In describing my invention reference will be made to the attacheddrawing wherein,

Figures 1 and 2 illustrate the two modifications of the modulatorsdescribed hereinbefore.

In Figure 1 amplitude sideband modulation With reduced carrier isaccomplished.

In the arrangement of Figure 2 phase and amplitude modulation ofsideband energy with reduced carrier is accomplished. 1

In both figures wave energy to 'be modulated is supplied from a highfrequency source of wave energy HF to a balanced relaying circuit L2 Z1Z2 and from the adjacent points of impedances Z1 Z2 by way of impedanceZ3 to the control grid 8 of an electron discharge tube V1 wherein thewave energy from the two paths are combined, repeated, or amplified.Grid bias for the control grid 8 is supplied by Z3 from a. source B2.The anode ll] of V1 is connected in an alternating current circuit C L3and is supplied by potential from a source B1. The screen grid electrodeof the tube may also be supplied by a potential from a source B1.By-passing condensers are connect ed between the screen grid and cathodeand between the low potential end of the alternating current circuit C1L3 and the cathode. The sideband energy and reduced carrier is suppliedfrom L3 to an inductance L4 in a load circuit A. Modulator tubes V2 V3have their anode to cathode impedances shunting the impedance Z1 Z2respectively. The anode to cathode direct current circuits of V2 V3 areboth completed through reactor X, source B3 andground. The alternatingcurrent anode to cathode circuits of V2 V3 are completed through Z1 Z2,the grid to cathode impedance'of tube V1 and impedance of Z3 inparallel, and ground G. Modulating potentials are supplied to the gridsl4 and 16 of tubes V2 V3 in phase opposition from the secondary oftransformer T, the primary of which is coupled to asource of modulatingpotentials AF.

In the modification of Figure 2 the impedances Z1 and Z2 are capacitiveand inductive in character respectively. The impedances Z1 Z2 are 5coupled with the terminals of L2 by blocking condensers B0. In thisarrangement phase and ampiitude modulated sidebands with reduced orsuppressed carrier are produced.

In Figure 1, tubes V2 V3 are the audio fre- 1 quency signal amplifiertubes. Tube V1 and circuit L2, Z1, Z2, and Z3 is the sideband modulatorcircuit in which the carrier frequency energy to be modulated fiows.Circuit HF and L1 is the carrier frequency energy source. Anelectrostatic shield S is placed between coils L1 and L2 to preventundesirable electrostatic coupling between the carrier frequency sourceand the modulator circuits. Reactor X allows the center point of coil L2to float at carrier frequency, that is, to be above ground carrierfrequency potential, This reactor X, as stated above, allows the directcurrent circuit to flow through the tubes V2 V3. The reactor X alsoinsures a steady flow of current from source B3 in that changes indirect current and/or potential therefrom are opposed by the impedanceof X, this improves the modulation characteristic of the entirearrangement because when the current in one tube, say V2, is reduced acertain amount (by modulation) the current in V3 is compelled toincrease by a like amount.

The operation of the modulators may be best described by referring toFigure 1. Assume all cathodes are heated and. all grids and anodes aresupplied with proper potentials. Carrier frequency energy is suppliedfrom source HF through coupling inductances L1 and L2 and from thelatter through impedances Z1 Z2 to the grid of the tube V3 and toimpedance Z3. If no audio signal or controlling potentials is present intransformer T the two sides of the circuit L2 Z1 Z2 are balancedrelative to ground by the equal impedances to ground of tubes V2 V3. Wehave assumed here that the impedances Z2 Z1 are balanced relative toground to which they are connected through Z3 and the control grid andcathode impedance of V1 in parallel. When signal potentials orcontrolling potentials are impressed from transformer T to the controlgrids of V2 and V3 then circuit L2 Z1 Z2 is unbalanced alternately, oneside being at high and the other side being at relatively low radiofrequency potential with respect to ground because of the relativechange in impedances of V2 V3. This results in the combination of thecarrier wave and the signal wave, which is impressed on the grid 8 oftube V1. The result is that tube V1 repeats or amplifies the sidebandenergy impressed on the grid 8 with the carrier wave energy reduced bycircuit L2 Z1 Z2.

Sideband energy with reduced or suppressed carrier appears in thecircuit C L3 and may be supplied to the load A. If resistance is usedfor impedance Z3 then tube V1 must be of the type that will pass anodecurrent without the presence of rectified current in grid I.

By replacing the similar impedances Z1 and Z2 of Figure 1 by dissimilarimpedances Z1 and Z2 as in Figure 2, a phase shift of the wave energysupplied by each branch to grid 8 with respect to the wave energy fromHF is accomplished and phase as well as amplitude modulation of thesidebands supplied to C L3 is produced with reduced or suppressedcarrier energy. In this case, a resistance is used at Z3 to supply thenecessary impedance.

In modifications, the tube V1 may be replaced by triodes or tubes havinga different number of electrodes. In the case triodes are utilized itmay be desirable to neutralize the effect of grid to anode capacity in awell known manner.

In both modifications the circuit C L3 may be connected to said load byWay of filters, amplitude limiters, or amplifiers, one or more of whichmay be used between the modulator stage V1 and the antenna.

I claim:

1. In a modulation system, a source of wave energy to be modulated, acombining tube having a control grid and a cathode, a reactance coupledto said source of wave energy, impedances of different charactercoupling said reactance to the control electrode of said tube, aconnection between the cathode of said tube and a point of symmetry onsaid reactance so that both sides of said circuit are substantiallybalanced, and means for unbalancing said circuit in accordance withsignal frequencies, whereby sideband energy, the amplitude and phase ofwhich varies in accordance with signal oscillations, and carrier energyof relatively small amplitude appears in the output circuit of saidtube.

2. In a modulation system, a source of Wave energy to be modulated, acombining tube having a control electrode and a cathode, a reactancecoupled to said source of wave energy, an inductive reactance and acapacitive reactance coupling the respective terminals of said firstreactance to the control electrode of said tube, a connection betweenthe cathode of said tube and a point of symmetry on said first reactanceso that both sides of said circuit are substantially relativelybalanced, and means for relatively unbalancing said circuit inaccordance with signal frequencies, whereby sideband energy, theamplitude and phase of which varies in accordance with signaloscillations, and carrier energy of relatively small amplitude appearson the control electrode of said tube.

3. In a signalling system, a source of wave energy of carrier wavefrequency to be modulated, an electron discharge tube having a controlgrid, a cathode and an anode, an output circuit coupled with the anodeand cathode of said tube, a reactance coupled to said source of waveenergy, an inductive reactance coupling a point on said first reactanceto said control grid, a capacitive reactance coupling a point on saidfirst reactance to said control grid, means coupling a point on saidfirst reactance to the cathode of said tube, said last point beingselected so that said reactance is substantially electricallysymmetrical with respect to said cathode, a pair of electron dischargedevices each having input and output electrodes, circuits connecting theoutput electrodes of said tubes in series with said first namedreactance, means connecting a point of electrical symmetry on said lastnamed circuit to the cathode of said first named tube, a source ofmodulating potentials, and means coupling said source of modulatingpotentials substantially in phase opposition to the input electrodes ofsaid pair of tubes whereby sideband energy modulated in phase andamplitude and carrier wave energy of relatively small amplitude isproduced in said output circuit.

GEORGE L. USSELMAN.

